Micro-gyroscopes are used in many applications including, but not limited to, communications, control and navigation systems for both space and land applications. These highly specialized applications need high performance and cost effective micro-gyroscopes.
There is known in the art a micro-machined electromechanical vibratory gyroscope designed for micro-spacecraft applications. The gyroscope is explained and described in a technical paper entitled "Silicon Bulk Micro-machined Vibratory Gyroscope" presented in June, 1996 at the Solid State Sensor and Actuator Workshop in Hilton Head, S.C.
The prior art gyroscope has a resonator having a "cloverleaf" structure consisting of a rim, four silicon leaves, and four soft supports, or cantilevers, made from a single crystal silicon. The four supports provide mechanical support and restoring force for the harmonic motion of the structure. A metal baton is rigidly attached to the center of the resonator, in a plane perpendicular to the plane of the silicon leaves, and to a quartz base plate spaced apart from the silicon leaves. The quartz base plate has a pattern of electrodes that coincides with the cloverleaf pattern of the silicon leaves. The electrodes include two drive electrodes and two sense electrodes.
The micro-gyroscope is electrostatically actuated and the sense electrodes detect Coriolis induced motions of the silicon leaves capacitively. The micro-gyroscope has a low resonant frequency due to the large mass of the metal post and the soft cantilevers. The response of the gyroscope is inversely proportional to the resonant frequency. Therefore, a low resonant frequency increases the responsivity of the device.
The leaves provide large areas for electrostatic driving and capacitive sensing. Applying an AC voltage to capacitors that are associated with the drive electrodes excites the resonator. This excites the rotation .crclbar..sub.x about the drive axis and rocking-like displacement .crclbar..sub.y for the leaves.
Because the post is rigidly attached to the leaves, the rocking movement of the leaves translates to movement of the baton. When the leaves oscillate in the drive mode, the displacement of the post is near parallel to the leaf surface in the y-direction. When a rotation rate is applied about the z-axis, Coriolis force acts on the oscillating post and causes its displacement in the x-direction. The baton displacement is translated back into the rocking motion, .crclbar..sub.y, of the leaves. The baton provides a large Coriolis coupling that transfers energy between the two orthogonal rocking modes.
The control electronics associated with the micro-gyroscope include an actuation circuit that is essentially an oscillator around the micro-gyroscope that locks onto the drive resonance mode. The signals from the sense electrodes are summed to remove the differential signal between them and the response of the sense resonance from the feedback loop. On the other hand, the sense circuit subtracts the signals from the sense electrodes to remove the common-mode drive signal.
The drive circuit includes an Automatic Gain Control (AGC) function to maintain a constant vibration amplitude and velocity for the micro-gyroscope. The AGC function requires a multiplier which in the prior art is accomplished using either expensive, analog integrated circuits that are not suitable for the harsh environment in space, or inexpensive Field Effect Transistors (FET's) operating in the triode region, also known as the variable resistance mode. While, FET's are capable of withstanding the harsh space environment, they are not uniform and have transistor to transistor variations that cause problems in the AGC loop. In addition, the integrated circuits and the FET's are subject to drift due to temperature and radiation exposure.
Another problem associated with prior art micro-gyroscopes is the potential for electrical interference that degrades gyroscope performance with regard to drift and scale factor stability. Micro-gyroscopes often operate the drive and sense signals at the same frequency to allow for simple electronic circuits. However, the use of a common frequency for both functions allows the relatively powerful drive signal to inadvertently electrically couple to the relatively weak sense signal. This is a disadvantage of having the benefit of simplified electronic circuit control. An alternative is to use a different frequency to carry the sense signal. However, this introduces a significant increase in the mechanical and electrical complexity of the circuit.
Noise and drift in the electronic circuit limit micro-gyroscope performance. Therefore, prior art micro-gyroscopes perform poorly and are unreliable in sensitive space applications.